Missing default hdri in shading option
Maybe you have enabled Scene World in the Shading options for the Viewport. In this case there is only an HDRI Preview when you actually use an HDRI in your world settings. If you disable Scene World, the Viewport shows a default HDRI. If this option is always there by default when opening a new scene, you have saved it to your startup file. To change it, when you're opening a new default scene, change the viewport option, then go to File &gt; Defaults &gt; Save Startup File.